1. Field of the Invention
This invention relates generally to hydraulic drives and, more particularly, to a continuously adjustable multiple range hydraulic drive.
2. Description of the Prior Art
Utility vehicles for agriculture, forestry or construction, as well as winches and cranes, are often driven by a hydraulic motor via a speed reduction gear or a step-down gear unit. For example, described in German Published Specification DE-OS 3137857 is a single-wheel drive unit in which a planetary gear unit driven by a hydraulic motor is arranged in a cup-shaped outer housing forming the hub of a vehicle. Drives of this configuration, however, achieve only relatively low transmission ratios when utilizing a variable displacement pump. This limitation is therefore particularly disadvantageous in situations where the utility vehicle must travel on public roadways, such as during movement between construction sites, because of the need to travel at the highest possible speed and thereby avoid causing traffic congestion.
In German Patent DE 39 07 633 A1, there is described a static displacement or hydrostatic drive having two hydraulic motors driven in parallel by a variable displacement pump. The hydraulic motors described therein, at least one of which is constructed so as to be shiftable, drive a single output shaft via an intermediate gear system. The output shaft drives the axle of a vehicle. A clutch, provided between the adjusting motor and the output, can be installed either between the intermediate gear and the output or between the adjusting motor and the intermediate gear.
With reference to the output, the gear ratio of the shiftable intermediate gear and that of the non-shiftable intermediate gear are selected differently in the prior art configurations mentioned above. While the transmission ratio of the non-shiftable intermediate gear of the present invention is roughly the same as that found in a conventional fixed displacement motor, it is considered essential to the present invention that the shiftable intermediate gear connected to the variable speed motor have a greater gear ratio. Known hydraulic motors are provided with fixed absorbing capacity and a gear reduction of the output shaft corresponding thereto. Moreover, in the known hydrostatic drives discussed above, the hydraulic motors themselves are arranged at both sides of the intermediate gear section. As such, a further disadvantage is present in that the intermediate gear and the axle drive require separate housings.
According to the process known from the prior art mentioned above, a rapid transition of the variable valve element of the variable speed hydraulic motor to the angle .alpha.=0 simultaneously results in a corresponding reduction of the swing-out angle of the drive pump. A large capital expenditure on control technology is required for the accurate coordination of these individual process steps.
A drive in which a pump drives a plurality of parallel hydraulic motors which are connected in turn individually with an intermediate gear is shown and described in U.S. Pat. No. 3,421,389. Each motor can be disconnected from the intermediate gear by a clutch and braked by brakes, In that patent, it is indicated that the individual stages may also be disconnected hydraulically, that is, by means of valves, instead of using the mechanical clutch and brakes. Neither the use of adjusting pumps, nor of adjusting motors, however, is known from this bulkily constructed drive. Moreover, the expensively outfitted drive requires a brake for braking the hydraulic motor precisely when the drive shaft of the respective motor is decoupled from the intermediate gear.
Accordingly, in hydraulic valve applications of the prior art, there has been no suggestion relating to means for preventing a speeding up of a hydraulically decoupled motor.
It is therefore an object of the present invention to provide a continuously adjustable multiple-range hydraulic drive which is simple and compact in construction, which utilizes a minimum of structural components, and which achieves a high conversion ratio while avoiding the disadvantages mentioned above.